1. Field of the Invention
The present invention relates to an improved cushioning system for athletic footwear which provides a large deflection for cushioning the initial impact of footstrike, a controlled stiffness response, a smooth transition to bottom-out and stability, and more specifically to a system which allows for customization of these response characteristics by adjustment of the orientation of a single bladder in a resilient foam material.
2. Description of Related Art
Basketball, tennis, running, and aerobics are but a few of the many popular athletic activities which produce a substantial impact on the foot when the foot strikes the ground. To cushion the strike force on the foot, as well as the leg and connecting tendons, the sole of shoes designed for such activities typically include several layers, including a resilient, shock absorbent layer such as a midsole and a ground contacting outer sole or outsole which provides both durability and traction.
The typical midsole uses one or more materials or components which affect the force of impact in two important ways, i.e., through shock absorption and energy dissipation. Shock absorption involves the attenuation of harmful impact forces to thereby provide enhanced foot protection. Energy dissipation is the dissemination of both impact and useful propulsive forces. Thus, a midsole with high energy dissipation characteristics generally has a relatively low resiliency and, conversely, a midsole with low energy dissipating characteristics generally has a relatively high resiliency. The optimum midsole should be designed with an impact response that takes into consideration both adequate shock absorption and sufficient resiliency.
One type of sole structure in which attempts have been made to design appropriate impact response are soles, or inserts for soles, that contain a bladder element of either a liquid or gaseous fluid. These bladder elements are either encapsulated in place during the foam midsole formation or dropped into a shallow, straight walled cavity and cemented in place, usually with a separate piece of foam cemented on top. Particularly successful gas filled structures are disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 to Marion F. Rudy, the contents of which are hereby incorporated by reference. An inflatable bladder or barrier member is formed of an elastomeric material having a multiplicity of preferably intercommunicating, fluid-containing chambers inflated to a relatively high pressure by a gas having a low diffusion rate through the bladder. The gas is supplemented by ambient air diffusing through the bladder to thereby increase the pressure therein and obtain a pressure that remains at or above its initial value over a period of years. (U.S. Pat. Nos. 4,340,626, 4,936,029 and 5,042,176 to Marion F. Rudy describe various diffusion mechanisms and are also hereby incorporated by reference.)
The pressurized, inflatable bladder insert is incorporated into the insole structure, in the '156 patent, by placement within a cavity below the upper, e.g., on top of a midsole layer and within sides of the upper or midsole. In the '945 patent, the inflatable bladder insert is encapsulated within a yieldable foam material, which functions as a bridging moderator filling in the irregularities of the bladder, providing a substantially smooth and contoured surface for supporting the foot and forming an easily handled structure for attachment to an upper. The presence of the moderating foam, however, detracts from the cushioning and perception benefits of the gas inflated bladder. Thus, when the inflated bladder is encapsulated in a foam midsole, the impact response characteristics of the bladder are hampered by the effect of the foam structure. Referring to FIG. 5 of the '945 patent for example, the cross-section of the midsole shows a series of tubes linked together to form the gas filled bladder. When the bladder is pressurized its tendency is to be generally round in cross-section. The spaces between those bladder portions are filled with foam. Because the foam-filled spaces include such sharp corners, the foam density in the midsole is uneven, i.e., the foam is of higher density in the corners and smaller spaces, and lower density along rounded or flatter areas of the bladder. Since foam has a stiffer response to compression, in the tighter areas with foam concentrations, the foam will dominate the cushioning response upon loading. So instead of a high deflection response, the response can be stiff due to the foam reaction. The cushioning effects of the bladder thus may be reduced due to the uneven concentrations of foam. In addition, the manufacturing techniques used to produce the sole structure formed by the combination of the foam midsole and inflated bladder must also be accommodating to both elements. For example, when encapsulating the inflatable bladder, only foams with relatively low processing temperatures can be used due to the susceptibility of the bladder to deform at high temperatures. The inflated bladder must also be designed with a thickness less than that of the midsole layer in order to allow for the presence of the foam encapsulating material completely therearound. Thus, there are manufacturing as well as performance constraints imposed in the foam encapsulation of an inflatable bladder.
A cushioning shoe sole component that includes a structure for adjusting the impact response of the component is disclosed in U.S. Pat. No. 4,817,304 to Mark G. Parker et al. The sole component of Parker et al. is a viscoelastic unit formed of a gas containing bladder and an elastomeric yieldable outer member encapsulating the bladder. The impact resistance of the viscoelastic unit is adjusted by forming a gap in the outer member at a predetermined area where it is desired to have the bladder predominate the impact response. The use of the gap provides an adjustment of the impact response, but the adjustment is localized to the area of the gap. The '304 patent does not disclose a way of tuning the impact response to optimize the response over the time of footstrike through the appropriate structuring of both the bladder and encapsulating material.
A cushioning system for a shoe sole which uses a bladder connected only along its perimeter and supported in an opening in resilient foam material, is disclosed in U.S. Pat. No. 5,685,090 to Tawney et al., which is hereby incorporated by reference. The bladder of Tawney et al. has generally curved upper and lower major surfaces and a sidewall that extends outward from each major surface. The angled sidewalls form a horizontally orientated V-shape in cross-section, which fits into a correspondingly shaped groove in the opening in the surrounding resilient foam material. Portions of the top and bottom of the bladder are not covered with the foam material. By forming the bladder without internal connections between the top and bottom surfaces, and exposing portions of the top and bottom surfaces, the feel of the bladder is maximized. However, the '090 patent does not disclose a way of tuning the impact response through design of both the bladder and foam material.
One type of prior art construction concerns air bladders employing an open-celled foam core as disclosed in U.S. Pat. Nos. 4,874,640 and 5,235,715 to Donzis. These cushioning elements do provide latitude in their design in that the open-celled foam cores allow for a variety of shapes of the bladder. However, bladders with foam core tensile members have the disadvantage of unreliable bonding of the core to the barrier layers. One of the main disadvantages of this construction is that the foam core defines the shape of the bladder and thus must necessarily function as a cushioning member at footstrike which detracts from the superior cushioning properties of air alone. The reason for this is that in order to withstand the high inflation pressures associated with such air bladders, the foam core must be of a high strength which requires the use of a higher density foam. The higher the density of the foam, the less the amount of available air space in the air bladder. Consequently, the reduction in the amount of air in the bladder decreases the benefits of cushioning. Cushioning generally is improved when the cushioning component, for a given impact, spreads the impact force over a longer period of time, resulting in a smaller impact force being transmitted to the wearer's body.
Even if a lower density foam is used, a significant amount of available air space is sacrificed which means that the deflection height of the bladder is reduced due to the presence of the foam, thus accelerating the effect of “bottoming-out.” Bottoming-out refers to the failure of a cushioning device to adequately decelerate an impact load. Most cushioning devices used in footwear are non-linear compression based systems, increasing in stiffness as they are loaded. Bottom-out is the point where the cushioning system is unable to compress any further. Compression-set refers to the permanent compression of foam after repeated loads which greatly diminishes its cushioning properties. In foam core bladders, compression set occurs due to the internal breakdown of cell walls under heavy cyclic compression loads such as walking or running. The walls of individual cells constituting the foam structure abrade and tear as they move against one another and fail. The breakdown of the foam exposes the wearer to greater shock forces, and in the extreme, to formation of an aneurysm or bump in the bladder under the foot of the wearer, which will cause pain to the wearer.
Another type of composite construction prior art concerns air bladders which employ three dimensional fabric as tensile members such as those disclosed in U.S. Pat. Nos. 4,906,502, 5,083,361 and 5,543,194 to Rudy; and U.S. Pat. Nos. 5,993,585 and 6,119,371 to Goodwin et al., which are hereby incorporated by reference. The bladders described in the Rudy patents have enjoyed commercial success in NIKE, Inc. brand footwear under the name Tensile-Air®. Bladders using fabric tensile members virtually eliminate deep peaks and valleys. In addition, the individual tensile fibers are small and deflect easily under load so that the fabric does not interfere with the cushioning properties of air.
One shortcoming of these bladders is that currently there is no known manufacturing method for making complex-curved, contoured shaped bladders using these fabric fiber tensile members. The bladders may have different levels, but the top and bottom surfaces remain flat with no contours and curves.
Another disadvantage is the possibility of bottoming-out. Although the fabric fibers easily deflect under load and are individually quite small, the sheer number of them necessary to maintain the shape of the bladder means that under high loads, a significant amount of the total deflection capability of the air bladder is reduced by the volume of fibers inside the bladder and the bladder can bottom-out.
One of the primary problems experienced with the fabric fibers is that these bladders are initially stiffer during initial loading than conventional air bladders. This results in a firmer feel at low impact loads and a stiffer “point of purchase” feel that belies their actual cushioning ability. The reason for this is because the fabric fibers have a relatively low elongation to properly hold the shape of the bladder in tension, so that the cumulative effect of thousands of these relatively inelastic fibers is a stiff feel. The tension of the outer surface caused by the low elongation or inelastic properties of the tensile member results in initial greater stiffness in the air bladder until the tension in the fibers is broken and the effect of the air in the bladder can come into play.
Another category of prior art concerns air bladders which are injection molded, blow-molded or vacuum-molded such as those disclosed in U.S. Pat. No. 4,670,995 to Huang; U.S. Pat. No. 4,845,861 to Moumdjian; U.S. Pat. Nos. 6,098,313, 5,572,804, and 5,976,541 to Skaja et al.; and U.S. Pat. No. 6,029,962 to Shorten et al. These manufacturing techniques can produce bladders of any desired contour and shape including complex shapes. A drawback of these air bladders can be the formation of stiff, vertically aligned columns of elastomeric material which form interior columns and interfere with the cushioning benefits of the air. Since these interior columns are formed or molded in the vertical position and within the outline of the bladder, there is significant resistance to compression upon loading which can severely impede the cushioning properties of the air.
Huang '995 teaches forming strong vertical columns so that they form a substantially rectilinear cavity in cross section. This is intended to give substantial vertical support to the air cushion so that the vertical columns of the air cushion can substantially support the weight of the wearer with no inflation (see '995, Column 5, lines 4–11). Huang '995 also teaches the formation of circular columns using blow-molding. In this prior art method, two symmetrical rod-like protrusions of the same width, shape and length extend from the two opposite mold halves to meet in the middle and thus form a thin web in the center of a circular column (see Column 4, lines 47–52, and depressions 21 in FIGS. 1–4, 10 and 17). These columns are formed of a wall thickness and dimension sufficient to substantially support the weight of a wearer in the uninflated condition. Further, no means are provided to cause the columns to flex in a predetermined fashion, which would reduce fatigue failures. Huang's columns 42 can be prone to fatigue failure due to compression loads, which force the columns to buckle and fold unpredictably. Under cyclic compression loads, the buckling can lead to fatigue failure of the columns.
Prior art cushioning systems which incorporate an air bag or bladder can be classified into two broad categories: cushioning systems which focused on the design of the bladder and its response characteristics; and cushioning systems which focused on the design of the supporting mechanical structure in and around the bladder.
The systems that focused on the air bladder itself dealt with the cushioning properties afforded by the pneumatics of the sealed, pressurized bladder. The pneumatic response is a desirable one because of the large deflections upon loading which corresponds to a softer, more cushioned feel, and a smooth transition to the bottom-out point. Potential drawbacks of a largely pneumatic system may include poor control of stiffness through compression and instability. Control of stiffness refers to the fact that a solely pneumatic system will exhibit the same stiffness function upon loading. There is no way to control the stiffness response. Instability refers to potential uneven loading and potential shear stresses due to the lack of structural constraints on the bladder upon loading.
Pneumatic systems also focused on the configuration of chambers within the bladder and the interconnection of the chambers to effect a desired response. Some bladders have become fairly complex and specialized for certain activities and placements in the midsole. The amount of variation in bladder configurations and their placement have required stocking of dozens of different bladders in the manufacturing process. Having to manufacture different bladders for different models of shoes adds to cost both in terms of manufacture and waste.
Certain prior pneumatic systems generally used air or gas in the bladder at pressures substantially above ambient. To achieve and maintain pressurization, it has been necessary to employ specially designed, high-cost barrier materials to form the bladders, and to select the appropriate gas depending on the barrier material to minimize the migration of gas through the barrier. This has required the use of specialty films and gases such as nitrogen or sulfur hexafluoride at high pressures within the bladders. Part and parcel of high pressure bladders filled with gases other than air or nitrogen is added requirement to protect the bladders in the design of the midsole to prevent rupture or puncture.
The prior art systems which focused on the mechanical structure by devising various foam shapes, columns, springs, etc., dealt with adjusting the properties of the foam's response to loading. Foam provides a cushioning response to loading in which the stiffness function can be controlled throughout and is very stable. However, foam, even with special construction techniques, does not provide the large deflection upon loading that pneumatic systems can deliver.